Voltage-source converters are available commercially which have a supply reversible converter, a voltage source and a load-side converter. The reversible converter comprises an inverse-parallel connection of two converters in three-phase bridge connection, with one converter being a supply converter responsible for controlling the power from the supply system to the load. The other converter is a regenerative converter responsible for controlling the power from the load to the supply system. For the load, for example, a polyphase machine is provided as a driving mechanism.
With a motor-operated polyphase machine, the supply converter is triggered in rectifier operation, whereby the power from the supply system reaches the load. In generator operation of the polyphase machine, the regenerative converter is controlled in inverter operation, whereby power is fed back to the supply system.
To prevent commutation failure in the regenerative converter, before the transition is made from motor to generator operation, the link voltage is lowered by a setpoint correction of the link voltage. The link voltage is lowered, for example, by 15%. However, in dynamic load changes, for example reversing operations, the link voltage cannot be lowered quickly by means of a setpoint correction. This means that the regenerative converter is triggered in inverter operation with an even higher link voltage. If the link voltage is too high, an overcurrent will occur which causes control signals from a control system to no longer reach the converter valves of the regenerative converter. Therefore, the conducting converter valves cannot supply the current to the follower converter valves of the regenerative converter. This means that the regenerative converter falls out of synchronism. Also, during triggering of the regenerative converter, an overcurrent can occur as a result of a system voltage dip when feeding back the power from the load to the supply system. As a result of the suppression of the control signals by the control, the regenerative converter will therefore fall out of synchronism.
This commutation failure is prevented in conventional voltage-source converters, because the voltage-source converters are always controlled in dynamic operation with lowered link voltage. The result is that the full power of the voltage-source converter cannot be utilized for the load.
Another way to avoid commutation failure is by coupling the regenerative converter of the reversible converter to the power supply with a high-performance transformer. In this way, the link voltage can be controlled to its nominal value. However, the high-preformance transformer and the reversible converter must be designed for a higher supply voltage.
There is thus a need for a circuit and a method to prevent commutation failure in a reversible converter without operating the voltage-source converter with lowered link voltage, and without having to design the reversible converter with a high-performance transformer for higher supply voltage.